In electrophotographic machines such as a zerographic copier, the documents that are copied may vary considerably in the actinic density of the paper substrate. White documents may have absolute densities that range from 0.04 to 0.20 depending on factors such as quality and age. Documents with colored or non-white backgrounds may have photo-topic densities as high as 0.50. In the absence of compensating techniques, a copier or printer designed to reproduce large areas responds to the absolute level of input density and hence, tends to produce unacceptable background when copying non-white background documents. Various compensating stabilization, are implemented to reduce the undesirable side effect of solid-area sensitivity. For example, it is known to provide manual background stabilization in the form of a range of copy-lighter/copy-darker settings on the control panel of a copier. The high background from a non-white original is suppressed by selecting a copy-lighter mode which typically increases exposure, or developer bias, or a combination of the two. This technique has the disadvantage of being a hit or miss technique and causes considerable delays in the copying operation until the exact compensation settings are found for the particular document.
It is also known to provide various forms of automatic background stabilization. For example, the Minolta 350 copier uses a passive developer-bias control that increases bias when the average image potential increases. The 3M "Sensitron" copier increases exposure when the average reflectance across the process decreases. Both of these systems compensate for variations in substrate density for low-area-coverage documents but, undesirably, also compensate for areas of high-image density, mistaking the image density for background density. Another example of an automatic background stabilization technique is that found in the Canon NP-270F copier. For this technique, the scanning system performs a prescan of the document, sensing the background image potential with a built-in electrometer. Exposure is then adjusted prior to the actual exposure scan. This technique results in undesirable loss of productivity due to the extra time required for the process.
It is also known, as disclosed in pending application U.S. Ser. No. 901,990 filed Sept. 2, 1986, a continuation of U.S. Ser. No. 778,119 filed Sept. 20, 1985, now abandoned, titled Automatic Background Stabilization for an Electrophotographic Printing Machine, to sample the charged potential along the leading edge margin of an exposed image frame and in response to the sample measurements to select a corresponding specific set of document density and development bias values stored in memory to adjust the density of the developer applied to the background position of the latent image.
It is also known that the Apeco Electric Eye copier discloses automatic exposure control in which light reflected from the original irradiates a small rectangular slit at the side of the optical system housing. This slit is covered with a sheet of standard white paper and the light diffusely transmitted by this paper is then incident on a cadmium sulfide photocell placed outside the housing directly behind the paper covered slit. The output of the photocell is used to vary to voltage applied to the exposure lamps and thus vary the exposure. The 3M 545 machine discloses a system in which a photo sensor in the light path near the lens senses the light reflected from the original. When the amplitude of the light decreases or increases, the system causes the exposure lamp voltage to change accordingly, thus automatically compensating for the type of original being copied. It is known in U.S. Pat. No. 4,372,674 to generate a bias voltage based upon the sensed density and color of the background of an original to control a developer apparatus. U.S. Pat. No. 4.050,806 discloses a plurality of sensing electrodes to sense the background area of an electrostatic image and to apply a biasing voltage to the developing electrode in response to the lowest value of the sensed potential, and U.S. Pat. No. 4,306,804 discloses using a reference surface for a portion of an original document produced by reverse optical scanning to sense the electrostatic potential on a photoconductive member to control the intensity of the light image of the original document or the developing bias voltage. In addition U.S. Pat. Nos. 4,200,391, 4,533,238 and 4,544,258 generally describe exposure control but not scorotron control in an attempt to eliminate a leading edge black band.
Difficulties with the above-described systems are that they often require prescanning of originals before copying and in those systems where adjustment is on the fly, a dark lead edge band may often appear on the copy. Also, in reproducing two up originals such as in a book mode or split scan mode, the sensor is not able to measure a signal corresponding to each of the two originals.
Accordingly, it is an object of the present invention to provide a new and improved background control that includes a sensor positioned within the optical path in order to view the original through the scanning optics to automatically adjust the illumination of the document on the fly. It is another object of the present invention to adjust the scorotron grid potential prior to scanning the original in response to a non-white document background to reduce the developability of low density originals and prevent a lead edge band from appearing on the copy. Further objects of the present inventions will become apparent as the following description proceeds and the features of novelty characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.